Effects of skin tone on photoacoustic imaging and oximetry

Abstract. Significance Photoacoustic imaging (PAI) provides contrast based on the concentration of optical absorbers in tissue, enabling the assessment of functional physiological parameters such as blood oxygen saturation (sO2). Recent evidence suggests that variation in melanin levels in the epidermis leads to measurement biases in optical technologies, which could potentially limit the application of these biomarkers in diverse populations. Aim To examine the effects of skin melanin pigmentation on PAI and oximetry. Approach We evaluated the effects of skin tone in PAI using a computational skin model, two-layer melanin-containing tissue-mimicking phantoms, and mice of a consistent genetic background with varying pigmentations. The computational skin model was validated by simulating the diffuse reflectance spectrum using the adding-doubling method, allowing us to assign our simulation parameters to approximate Fitzpatrick skin types. Monte Carlo simulations and acoustic simulations were run to obtain idealized photoacoustic images of our skin model. Photoacoustic images of the phantoms and mice were acquired using a commercial instrument. Reconstructed images were processed with linear spectral unmixing to estimate blood oxygenation. Linear unmixing results were compared with a learned unmixing approach based on gradient-boosted regression. Results Our computational skin model was consistent with representative literature for in vivo skin reflectance measurements. We observed consistent spectral coloring effects across all model systems, with an overestimation of sO2 and more image artifacts observed with increasing melanin concentration. The learned unmixing approach reduced the measurement bias, but predictions made at lower blood sO2 still suffered from a skin tone-dependent effect. Conclusion PAI demonstrates measurement bias, including an overestimation of blood sO2, in higher Fitzpatrick skin types. Future research should aim to characterize this effect in humans to ensure equitable application of the technology.

Figure S6.Varying blood oxygenation in the phantom tubing over time.The true blood oxygenation, as measured by a pO2 probe and converted to sO2 using the Severinghaus equation, is compared to the photoacoustic estimates of sO EST 2 using linear unmixing and learned unmixing for each level of melanin concentration (A) 0.0 mg mL 1 (b) 0.1 mg mL 1 and (C) 0.21 mg mL 1 .Figure S8.Spectral unmixing results when applied to the optical simulations of a blood-flow phantom reveal biases due to spectral colouring.Linear unmixing of the initial pressure distributions reveals a melanin concentration-dependent bias with both learned and linear unmixing (A).Colour scale corresponds to melanosome concentration.Photoacoustic blood oxygenation estimates increase with melanosome volume percentage in the outer layer (B).
Figure S9.Increased pigment in the mouse skin leads to increased photoacoustic signal in the skin and decreased photoacoustic signal in the body.B6 mice have a significantly higher photoacoustic signal at 700 nm in the skin than albino mice (p=0.010) and pigmentation significantly increases this further (p < 0.001) (A).Non-pigmented B6 mice do not di↵er in their photoacoustic signal in the body (p = 0.95), however, pigmentation significantly reduces the photoacoustic signal in the body (p < 0.001) (B).

Figure S2 .
Figure S2.Schematic diagram of the 3-D printed phantom mould.A cross-section through the centre of the cylindrical mould.Left: the base of the mould, with Luer lock attachment for a needle and plastic tubing attachment, which is placed in the centre of the phantom.Right: a cross-section through the cylindrical mould, with a lower compartment in which the base mixture of the phantom is poured, and an upper compartment in which the melanin mixture is poured.

Figure S3 .
Figure S3.Example image of B6 mice assigned to non-pigmented and pigmented group.(A) An example of a non-pigmented mouse (prone and supine).(B) An example of a pigmented mouse (prone and supine).

Figure S4 .
Figure S4.Full optical and acoustic simulations show decreased signal and spectral colouring with increasing Fitzpatrick skin type, consistent with purely optical simulations.(A) Photoacoustic reconstructed images, unmixed total haemoglobin (THb), and unmixed blood oxygenation (sO EST 2 ) at each Fitzpatrick type for fully oxygenated blood in the forearm model.(B) Epidermis photoacoustic signal rises with increasing Fitzpatrick type at all wavelengths.(C) The photoacoustic signal at 700 nm increases with increasing Fitzpatrick type in the epidermis.(D) Increased spectral colouring is observed in the normalised blood photoacoustic spectra and (E) the photoacoustic signal at 700 nm decreases with increasing Fitzpatrick type in the blood vessel.

Figure S5 .
Figure S5.Double integrating-sphere measurements confirm expected optical absorption and scattering properties of skin-mimicking phantoms.Optical absorption (A) and scattering (B) coe cients as a function of wavelength were calculated using the inverse adding-doubling method.Shaded areas show 95 % confidence intervals.(C) Photographs of the skin-mimicking phantom material show qualitative agreement with the higher Fitzpatrick skin types.

Figure S7 .
Figure S7.Optical simulations of blood-flow phantoms reveal decreased blood signal with increasing melanin concentration and spectral colouring in the blood spectra.(A) Initial pressure, and linear unmixing total haemoglobin and blood oxygenation, sO EST 2 in representative simulations.(B) Initial pressure spectra of the melanin layer as a function of wavelength and (C) at 700 nm as a function of melanosome volume percentage.(D) Mean-normalised initial pressure spectra of the blood as a function of wavelength, and (E) non-normalised initial pressure spectra of the blood at 700 nm as a function of melanosome volume percentage.

Figure S10 .
Figure S10.spectral decolouring performs poorly with in vivo mouse imaging data.Linear unmixing (top) is compared to learned unmixing (bottom) in albino mice, non-pigmented B6 mice and pigmented B6 mice.